"Mushiness" of SCT Views

[beka]

Hi All,

I have heard of that SCT views are "soft"  or "mushy" several times on the forums. I was curious about this because I could not really imagine the views through my SCT being any crisper. On globulars and open clusters, if the faint stars were any more pin point then I wouldn't see them (and I don't use glasses). So I tried to simulate the Point Spread Functions (PSF)  for different apertures of scopes shown in the image below, and this is my take on it...

The curve for the lowest aperture is the flatest Purple curve, the next flattest Green curve has double the aperture, and the tallest Blue curve has three times the aperture of that of the Purple. If the threshold intensity that our eye can see is at the Red horizontal line, we can see that the width of the PSF is larger for the Blue curve than that of the smaller aperture Green - meaning it will appear more "bloated". We can also have a similar situation between the Purple and Green a little lower down.

Thus for certain stars the image would appear more pinpoint in a scope of smaller diameter than that for a larger one. Together with the larger image scale of SCTs, and the fact that larger apertures are more susceptible to seeing conditions, this could explain why we might perceive SCT images as soft, bloated or mushy.

Best

 

 

 

 

 

Edited September 9, 2022 by beka
A correction...

[Andy ES]

35 minutes ago, beka said:

I could not really imagine the views through my SCT being any crisper.

Beka

I agree with you 100%.

Again I have been a bit confused by the “mushy” comments I have read.

I've been very satisfied with my set up so far.

I won’t pretend to understand what your graph means, but at the eyepiece it all looks fine! (Perhaps I don’t know any better)😁

Edited September 9, 2022 by Andy ES
Spelling again

[dunc]

My old Meade 8" LX90 UHTC circa 2006 is the epitome of mushiness despite every tweak and adjustment possible. Mind you I was awed by it when I got it (my first real scope). I suspect the quality of the average SCT has been extremely variable over the years. The bigger more expensive lower volume SCTs seem to fare better. I've just got a TS  8"Classsic Cassegrain and its soooo much better. I have a TS1506UNC f4 newt, Sharpstar 61EDPH, TAL2 1200mm f7 newt and SW AT70 travel scope  as well for comparison, they are all better than the Meade!

[Cosmic Geoff]

I have often seen reports that SCT views are 'mushy' with the implication that these instruments are inferior to other designs.  Following a recent experience I now suspect that the issue could be poor collimation. I have two SCTs - a C8 SE (quick deploy) and a CPC800 (for serious business).

I had a feeling that the C8 SE imaged better, yet the CPC800 resolved double stars OK.  The views through the C8 SE always looked perfectly fine, with suitably positioned stars showing an Airy disc and rings. But was the view of doubles through the CPC800 always a little untidy?  

Eventually, I carried out a side-by-side trial, swapping over the diagonals and eyepieces to eliminate these as a possible cause of trouble. Result: the C8 SE was definitely performing better.  The only possible conclusion: either the CPC800 was a 'Friday Afternoon' job or the collimation was slightly out.  So I aimed it at Polaris and slackened one of the secondary aligning screws by a fraction of a turn. Ugh, worse.  I slackened an opposing screw slightly and that definitely improved matters. So left it like that, and tried some planetary imaging with it within hours.  Result - significantly better than before, and pretty much like the kind of result I sometimes got from the C8 on the less stable SE mount.

So, a very slight mis-collimation of a SCT significantly affects the performance.

[michael.h.f.wilkinson]

They are very sensitive to collimation, but when properly collimated they can be very good indeed. They are fairly simple to collimate, and hold collimation very well indeed. Due to the relatively large secondary obstruction, contrast will be a bit softer, but resolution is good. Typically Edge-HD  versions are better corrected (and not just at the edge). Having said that, I am still very happy with the views and images with my 1995 GP-C8, now almost 27 years old

[Mr Spock]

My C9.25 was fine. Collimation did have to be exact and it was very susceptible to poor seeing conditions. 
At its best it easily split a 0.7” double with space to spare. 
SCTs do gave a poor MTF compared to other designs. This can make Jupiter’s belts fainter than Newts, Maks and refractors for example. 

[Elp]

I get the mushiness having started with newtonians and refractors. Isn't internal temperature also a factor on top of collimation? A Newtonian has central obstructions too but I find them to be sharper overall with more contrast. A refractor more so. Yet my C6 seems mushy, but the increased focal length surely also makes a difference creating larger FWHM stars?

[michael.h.f.wilkinson]

2 hours ago, Elp said:

I get the mushiness having started with newtonians and refractors. Isn't internal temperature also a factor on top of collimation? A Newtonian has central obstructions too but I find them to be sharper overall with more contrast. A refractor more so. Yet my C6 seems mushy, but the increased focal length surely also makes a difference creating larger FWHM stars?

Newtonians typically have smaller CO than SCTs. I had a 6" F/8 "planet killer" Newtonian, with just 23% CO compared to about 34% for the C8. The C8 captures more detail, but the contrast is a bit lower. The point on internal temperature is very important. A Newtonian is an open design, which cools down to ambient temperatures much quicker than the closed-tube SCTs and Maks. Big refractors have similar problems. There is also this: very people have looked through a apochromatic refractor larger than about 6". The biggest I have looked through was Olly's TEC 140. Great scope which gave me horrible views of Jupiter. Nothing to do with the scope, everything to do with seeing (Jupiter was less than 10 degrees above a horizon filled with hills that had been baking in summer heat).  Bigger scopes suffer far more from bad seeing than smaller ones, simply because the seeing disk is much bigger than the Airy disk of the optics.

[RobertI]

Interesting post, I’ll take your word for the logic of your analysis! I think ‘mushiness’ is too vague a term to really understand what problems people are seeing. It seems like there are many problems. My personal experience with my C8 is that bright stars are not always pinpoint and can seem a bit ‘hairy’ (I think @michael.h.f.wilkinson coined this term - I like it!). This makes certain doubles with bright primaries difficult to split and it means I tend to avoid using the scope for double splitting, probably unreasonably so. I think part of the problem is that SCTs have such long focal lengths you can often forget how high a power you are observing. I’ve also found that low contrast planetary features are not as distinct as you would hope from the aperture. But I’ve never really noticed any kind of ‘mush’ types issues on the moon or clusters. Galaxies and nebulae are mushy anyway! 🙂

Edited September 9, 2022 by RobertI

[vlaiv]

5 hours ago, beka said:

The curve for the lowest aperture is the flatest Purple curve, the next flattest Green curve has double the aperture, and the tallest Blue curve has three times the aperture of that of the Purple. If the threshold intensity that our eye can see is at the Red horizontal line, we can see that the width of the PSF is larger for the Blue curve than that of the smaller aperture Green - meaning it will appear more "bloated". We can also have a similar situation between the Purple and Green a little lower down.

Thus for certain stars the image would appear more pinpoint in a scope of smaller diameter than that for a larger one. Together with the larger image scale of SCTs, and the fact that larger apertures are more susceptible to seeing conditions, this could explain why we might perceive SCT images as soft, bloated or mushy.

Except for the fact that our vision does not work like that.

There is no hard cutoff and our perception is not linear so you can't equate linear intensity with our perception.

What you have shown in that graph is more related image processing and setting of the white point and bloated stars then to visual side of things.

As for SCTs being mushy - there are really two explanations:

1. collimation / thermal issues

2. Spherical aberration

First is self explanatory, and second depends both on quality of the optics - but also on primary to secondary distance. However, most SCT designs focus by changing distance of primary to secondary, and what sort of accessories are used will determine amount of in/out focus required.

Same SCT scope can become mushier if longer optical path is used - like binoviewers or different type of diagonal then what the scope was designed for. Even barlow lens will alter focus position.

SCTs have spherical primary mirror and have rather large corrector plate in the front. If that corrector plate is not figured properly - there will be residual spherical aberration. Add to that substantial central obstruction and the fact that it is F/10 scope with long focal length that makes high powers very accessible - and mushiness best shows at high powers.

Sometimes it is hard to distinguish real source of mushiness (even seeing can be part of it) - but mushiness itself is easy to identify - hence numerous accounts of it.

[Andy ES]

Can I please ask what mushiness actually is, or how does it manifest its self.

As my set up seems fine 🤔

[Cosmic Geoff]

6 hours ago, RobertI said:

My personal experience with my C8 is that bright stars are not always pinpoint and can seem a bit ‘hairy’ (

Sounds like my CPC800 before I tweaked the collimation.  My C8 SE did not produce 'hairy' stars.  

[RobertI]

20 minutes ago, Cosmic Geoff said:

Sounds like my CPC800 before I tweaked the collimation.  My C8 SE did not produce 'hairy' stars.  

Interesting, I’ll do some proper tweaking and testing to see what I can do, it would be great to find out I can get pinpoint stars with the scope, although I think mirror flop may end up spoiling the party. 

[CraigT82]

8 hours ago, michael.h.f.wilkinson said:

Bigger scopes suffer far more from bad seeing than smaller ones, simply because the seeing disk is much bigger than the Airy disk of the optics.

Isn’t the Airy disk size related to the focal ratio only, rather than aperture size? Eg. 200mm f/10 scope will produce an airy disk the same size as a 100mm f/10 scope, and so the seeing blur relative to Airy disk diameter should be identical for both? 
 

Edit: Nope I’m talking rubbish. The diameter at the focal plane would be the same but the angular size subtended in the sky would be half for the larger scope. 

Edited September 9, 2022 by CraigT82

[vlaiv]

47 minutes ago, CraigT82 said:

The diameter at the focal plane would be the same but the angular size subtended in the sky would be half for the larger scope. 

Yep, projection size vs angular size.

[Louis D]

3 hours ago, Andy ES said:

Can I please ask what mushiness actually is, or how does it manifest its self.

Stars don't focus to pinpoints.  They're a bit bloated at best focus compared to the view through a refractor or Newt under the same conditions.  Planets don't manifest much distinct detail as compared to other designs.  This SCT mushiness becomes painfully obvious at star parties if you take the time to walk around the observing field and check out the views through a multitude of scopes all dealing with the same conditions.  There's simply no way every SCT is out of collimation when so many advanced observers are about.  Only the Edge HD design has shown sharp images rivaling Newts and refractors in my experience.  As such, I'm going to chalk the mushiness up to uncorrected spherical and other aberrations in a standard SCT.

[WJC]

So far, on this thread, I have seen talk of aperture and focal length. “Mushiness” is NOT an optical term. You should be talking about contrast and resolution. That would tell you a lot. Not only would your view be affected by temperature and humidity. It would be wildly affected by the size of the secondary!

If you think you have pinpoint stars, that thought would be out the window by taking a photo with an 8-inch SCT and another with a 6-inch apochromatic refractor. Everything is relative. Too many people blame “anomalies” in viewing on the instrument without considering obstruction size, weather conditions, physiological considerations, and what they don’t know about optics ... on the scope.  

[Louis D]

Just saying, when I take a statistical cross section of an observing field of dozens of scopes of all types, and the SCTs are putting up the least pleasing images regardless of aperture, I'm going to blame the design.  After attending multiple public star parties in the 90s, it was clear enough to me that I took a hard pass on buying an SCT and instead bought a Dob (Newtonian).  A Dob simply wasn't what I thought of as a desirable telescope as a newbie (big, dumpy, awkward, undriven, yard cannon, etc.), but multiple comparisons over several years lead me to that decision.  I'm still using that scope 24 years later on a regular basis.  I've since added an achromat, an ED, an APO, a big Dob, and a Mak, but I still love that midsized Dob all these years later.  Other than an EdgeHD, I haven't seriously considered getting an SCT of any size in those 20+ years.

[WJC]

Having been in military and civilian optics for over 50 years, and owned telescope of all sizes and configurations, you will find no dispute from me. But I was amazed that is all the words so far, no one even mentioned the size of the secondary obstruction.

[Sunshine]

This may have been posted once before as I vaguely recall, I'm not bashing SCT's as there are millions of them out there with happy users.

The cartoon is funny, though.

 

[MartinB]

The secondary impacts on contrast.  If seeing is poor I prefer to use my 120mm doublet refractor on planets or the moon but if the seeing is good my 10" SCT is far superior.  Careful collimation of the secondary makes a big difference.

[RobertI]

4 hours ago, Louis D said:

Just saying, when I take a statistical cross section of an observing field of dozens of scopes of all types, and the SCTs are putting up the least pleasing images regardless of aperture, I'm going to blame the design.  After attending multiple public star parties in the 90s, it was clear enough to me that I took a hard pass on buying an SCT and instead bought a Dob (Newtonian).  A Dob simply wasn't what I thought of as a desirable telescope as a newbie (big, dumpy, awkward, undriven, yard cannon, etc.), but multiple comparisons over several years lead me to that decision.  I'm still using that scope 24 years later on a regular basis.  I've since added an achromat, an ED, an APO, a big Dob, and a Mak, but I still love that midsized Dob all these years later.  Other than an EdgeHD, I haven't seriously considered getting an SCT of any size in those 20+ years.

I have to say, if I knew back in 1999 what I know now, I wouldn’t have bought an SCT. As I mentioned in a separate thread, it was the scope I had always wanted as a kid back in the early 80’s (as did many people) and when I resurrected my hobby I bought my ‘dream’ scope without really doing any research. Having said that, nearly 25 years later, it has stayed with me through numerous house moves, accompanied me on numerous holidays, given me some truly memorable views and now sits alongside my new 102ED when I want to do ‘dual’ observing. I guess we make the best of what we have!

[beka]

18 hours ago, vlaiv said:

Except for the fact that our vision does not work like that...

Our vision will have an intensity threshold for some particular situation, though of course will vary from time to time and from person to person - for example if dark adapted etc. What I was attempting to illustrate was that if  the threshhold for a particular observer at the time of observation (no suggestion that it is linear) is at the level of the red line then it is possible that the size of the disk seen for a certain star might be bigger for a scope of larger aperture vs one with a smaller aperture - which we might interpret as bloat.

 

11 hours ago, CraigT82 said:

Isn’t the Airy disk size related to the focal ratio only, rather than aperture size? Eg. 200mm f/10 scope will produce an airy disk the same size as a 100mm f/10 scope, and so the seeing blur relative to Airy disk diameter should be identical for both? 
 

Edit: Nope I’m talking rubbish. The diameter at the focal plane would be the same but the angular size subtended in the sky would be half for the larger scope. 

Hi CraigT82, actually the size of the airy disk depends only on the aperture and not the focal length for a point source like a star.

Edited September 10, 2022 by beka

[Mr Spock]

In terms of optical 'defects', rather than seeing, cooling, collimation etc, it's what I was alluding to earlier with the comparison on Jupiter's belts. With an SCT the belts will appear fainter than with an apo refractor. I state apo refractor because an achromat, as well as chromatic aberration, will also often suffer from spherical aberration which has a similar effect.

The central obstruction has an effect on the MTF v spacial frequency:

This doesn't affect the optic's resolution, just how local contrast is perceived. Imaging can correct for this which is why you see so many top planetary imagers using C14s for example. 

'Mushiness' is a subjective descriptive word which could mean a whole number of things. You could say 'softness', but that is also an inaccurate descriptive word.

When people say contrast, they are generally referring to spacial contrast rather than absolute contrast. Contrast is the difference between the brightest and darkest areas in an image. So when someone says a 100mm refractor has better contrast than an 8" SCT, they are incorrect. Black is black in both instruments, but the 8" clearly has brighter whites - more contrast not less! However, due to the effect of the central obstruction on the MTF (see curves above), the refractor has better spatial contrast than the 8" - the Jupiter's belts scenario described above.

The other contributing factor, optically, is the quality of the optics. A ⅛ wave objective will put more light into the airy disc than an objective with ¼ wave; this will give better sharpness and spacial contrast and appear less 'soft'. But that is a whole new topic in itself.

 

[vlaiv]

2 hours ago, beka said:

Our vision will have an intensity threshold for some particular situation, though of course will vary from time to time and from person to person - for example if dark adapted etc. What I was attempting to illustrate was that if  the threshhold for a particular observer at the time of observation (no suggestion that it is linear) is at the level of the red line then it is possible that the size of the disk seen for a certain star might be bigger for a scope of larger aperture vs one with a smaller aperture - which we might interpret as bloat.

There is no hard limit to what we can detect - no red line which represents "above" and "below".

https://en.wikipedia.org/wiki/Just-noticeable_difference

Quote

a just-noticeable difference or JND is the amount something must be changed in order for a difference to be noticeable, detectable at least half the time (absolute threshold).

With visual astronomy, you will often read reports of target appearing and fading out of view. It will become stably visible after certain amount of time - as we build up mental image of it, sort of train our brain to see it. This is combination of two effect - JND and the fact that we can "burn" certain image in our brain.

Further more, besides JND - there is issue of sampling, particle nature of light and the way our brain interprets things.

We don't see continuous signal although it might seem so to us. We see in pixels - or rather special neurons that are evolved to sense light. These are finite in size and are arranged in irregular grid.

Particle nature of light means that light is not continuous signal, but rather arrives in photons and thus have associated Poisson noise.

We can detect very faint signals - just 7-9 photons strong. At that photon rate - noise will be very big, however we never seem to see such noise. This is because of how our brain works in order for us to see something faint. Several criteria must be fulfilled for sensation to happen.

Few adjacent photo receptors must be triggered, and then brain decides on some threshold to produce actual signal - but denoised.

Then there is matter of magnification. You need to increase magnification in order to start resolving Airy disk of source star. To a certain point - star is just point source, and two point sources look the same to our eye - or rather have same shape on our retina that depends on "optics" of our eye. Eye lens distorts point of light just enough so it covers few receptor cells in order to be detectable - and we see it as point because our brain filters things.

When we start increasing magnification - we reach place where airy disk is no longer point source and is resolved. This spreads the light over more surface and reduces photon count. If star is already at threshold visibility - further increase of magnification will push it below that threshold (here threshold again is not clean line but about +/- 7% of intensity and depends on how long you stare at the thing).

All of this shows that you can't explain things with such a rather simple model as constant cutoff point.